2/2 seat valves are valves with two inlets and/or outlets and two switch positions, the inlets or outlets being joined to each other in one (open) switch position and separated in the other (closed) switch position. In the case of the valves to be considered in greater detail here, the valves may be monostable valves, i.e., valves which in the de-energized condition are either open or closed.
The opening or closing of solenoid valves is accomplished by the activation or deactivation of an (electro-) magnet provided in the valve.
The activation and deactivation of the electromagnet, more precisely, the energizing of the coil of the electromagnet, results in the movement, i.e., the attraction or the release of an armature which is connected with the closure element of the valve and thereby moves the closure element with it.
Although valves of this kind originally were provided "only" to switch back and forth between the two switch positions, they recently have also been operated proportionally. Thus they are operated so that the closure element assumes an intermediate position between the switch positions, or they are opened and closed in rapid sequence for equal or varied periods of time so that in the process a condition corresponding to a stationary intermediate position of the closure element develops.
One possible application of this type of valve actuation is the setting of a defined differential pressure across the valve (for example in the aforementioned ABS hydraulic assemblies). Since the dynamic response of the valve, however, is strongly nonlinear, this is fairly difficult to arrange.
Regulation concepts which have been developed in the past for the modulation of pressure using the aforementioned valves are based on linear pressure regulation systems with a pre-control system with a differential pressure working point being set by the pre-control system around which the regulation takes place.
The basic design of a regulation system of this kind is illustrated in FIG. 3.
The valve which is to be actuated in the arrangement per FIG. 3 is designated with reference number 1 and the regulating circuit through which the valve is to be actuated is designated with reference number 2. Regulating circuit 2 includes pre-control unit 21 and regulating unit 22.
The valve characteristic is stored in pre-control unit 21. As a result, pre-control unit 21 is able, depending on a target differential pressure (.DELTA.P.sub.target) which is supposed to develop above the valve, to generate a control signal through which valve 1 normally is actuated in such manner that the actual differential pressure (.DELTA.P.sub.actual) that develops across the valve corresponds to the target differential pressure or at least approaches it relatively closely. If needed, existing deviations between the target differential pressure and the actual differential pressure are compensated by regulating unit 22. Regulating unit 22 generates, on the basis of the difference between the target differential pressure and the actual differential pressure, a regulating signal which is summed with the control signal generated by pre-control unit 21.
Because the actual differential pressure which develops above valve 1 approaches relatively closely the target differential pressure through pre-control unit 21 alone, regulating unit 22 can be a linearly functioning regulating unit and accordingly be of relatively simple construction; through pre-control unit 21, a tentative operating point develops around which linear regulation can take place.
Experience shows that regulating circuit 2, despite its apparently good suitability for valve actuation, at times is not able to bring the actual differential pressure quickly, precisely, and lastingly to the target differential pressure. The cause of this is in particular the strong dependency of the valve characteristic on temperature.
With increasing temperature of the coil of the electromagnet of valve 1 and conditions unchanged otherwise, the current flowing through the coil decreases and, as a result, the force exerted on the closure element of the valve also decreases and the closure element can no longer withstand relatively great pressure differentials.
This effect can be compensated up to a certain degree by regulating unit 22. At greater temperature fluctuations, however, special measures are required for the elimination of these effects.
The effects elicited by temperature fluctuations can be compensated by providing a secondary current regulation system. In this case, a current-regulating unit (not shown in FIG. 3), the task of which is to regulate the (actual) current flowing through the valve to the particular target current, is inserted between valve 1 and regulating circuit 2.
By this means valve 1 can be operated according to specifications independently of temperature. However, provision of a secondary current control system comes at a considerable technological cost. This applies especially for determining the actual current flowing through the valve (conversion from a voltage drop which develops across a calibrated precision resistor, more or less frequent adjustment of the measuring device, etc.).